Nearly all high peak power USP laser systems use the technique of chirped pulse amplification (CPA) to produce short-duration high-intensity pulses. Chirped pulse amplification increases the energy of a short pulse while avoiding optical amplifier damage. In this technique, the duration of the pulse is increased by dispersing it temporally as a function of wavelength (a process called “chirping”), amplifying the chirped pulse, and then recompressing the chirped pulse to significantly shorten its duration. By lengthening the pulse in time, energy can be efficiently extracted from an optical amplifier gain medium while the peak power levels of the pulse are below the damage threshold of the optical amplifier.
Typically, the amplifiers in a CPA system are configured to operate at an optimal repetition rate. The optimal repetition rate is often the lowest possible rate at which significant Amplified Spontaneous Emission (ASE) is prevented. The lowest rate at which ASE is prevented is referred to as the ASE-limiting rate. This rate is determined by physical properties of the amplifier, and is approximately equal to the inverse of the ASE lifetime of the gain medium. Below the ASE-limiting rate, energy within the amplifiers is lost as ASE. Above the ASE limiting rate, the amplifiers may not have time to fully recharge between pulses. Therefore, the gain of the amplifier is typically highest when the operating at the ASE-limiting rate.